The present invention relates to an electrophotographic recording material which contains an electrically conductive substrate and a photoconductive layer of amorphous silicon and hydrogen applied to the substrate, and to a method for producing such an electrophotographic recording material.
Amorphous silicon layers for indirect electrophotography have a mechanical hardness and heat resistance which is much greater than that of prior art record carriers. At the same time, such layers exhibit sensitivity over a broad spectrum and a level of sensitivity which, in almost the entire visible range, lies above that of materials presently employed in practice. The use of amorphous silicon leads to a significant improvement in copying machines with respect to the service life of the photoconductors and the copying speed. Moreover, amorphous silicon is a nontoxic material and thus excellently environmentally compatible.
The production of electrophotographic layers of amorphous silicon can be effected by means of two methods. In the past, production by means of a silane glow discharge has been used most frequently. In this process, the silane gas, monosilane or higher order silane, is decomposed in a high frequency plasma discharge, with an amorphous silicon hydrogen alloy being precipitated on heated substrates. The hydrogen is necessary for the realization of good electrical and optical characteristics. In order to produce electrophotographically suitable layers, it is known to add a small amount of diborane to the silane atmosphere, and possibly also oxygen. See DE-OS No. 3,117,035. This manufacturing process requires the use of highly toxic, easily flammable gases and gas mixtures. The toxicity of diborane, in particular, is expressed in its very low maximum workplace concentration value of 0.1 ppm. Although the resulting layers are no danger to health, since they are hard solid state layers, extensive and thus costly measures must be taken during the manufacturing process for handling the above-mentioned gases as well as for removing the gas mixtures discharged from the coating apparatus.
One process which does not require the use of health endangering gases, such as silane or diborane, is the cathode sputtering process. In this process, the ionized gas atoms from a noble gas plasma discharge, generally employing argon, eject particles from a solid silicon target (cathode) which precipitate in a layer on the heated substrate. The hydrogen required to realize suitable properties is mixed in with the noble gas, with the sputtering, in contradistinction to the silane glow discharge process, permitting a variation in the hydrogen content and thus in the properties of the resulting coatings. See T. D. Moustakas, J. Electr. Mater. 8, 391/1979.
It is known that electrophotographic record carriers of amorphous silicon can be produced with the sputtering process. However, only in multiple layer arrangements and by varying the hydrogen component as well as by including SiO bonds in one of the partial layers, as disclosed in European Pat. No. 0045204, and possibly by subsequently coating the record carrier with a covering layer will such record carriers exhibit electrophotographic usefulness.
Both manufacturing processes, sputtering and glow discharge, differ principally in the composition of their gases and in the kinetic energies of the gas molecules, which are determined by the gas pressure. Therefore, the coatings produced with these two processes also differ noticeably in their solid state characteristics, such as, for example, charge carrier mobility or hydrogen inclusion. Moreover, it cannot be expected that, for example, the doping properties in both processes are the same.
Although it is known that in the silane glow discharge process, high specific resistances can be realized by adding oxygen, (E. Holzkampfer, J. Stuke, R. Fischer, 4th Photovoltaic Solar Energy Conference, Stresa (1982), Ed. W. H. Ploss, G. Grassi, D. Reidel Publ. Comp.), with large amounts of oxygen the spectral sensitivity range shifts in a disadvantageous manner, due to the formation of SiO.sub.x, to much shorter wavelengths. With smaller amounts of added oxygen in the glow discharge process, high resistances can be realized only by adding diborane. See DE-OS No. 3,117,035. Large amounts of oxygen in the sputtering atmosphere, however, and thus proportions greater than 1 at% in the coatings, result in a decrease in resistance and thus in coatings which are unusable for electrophotographic purposes See, B. G. Yacobi, R. W. Collins, G. Moddel, P. Viktorovitch and W. Paul, Phys. Rev. B 24, 5907 (1981). Here it becomes evident how different the effects are of the same amounts of added gas in both processes. Without the addition of oxygen, only dark resistances in the range from 10.sup.10 to 10.sup.11 ohm cm could be realized by sputtering of amorphous silicon, values much too low for electrophotographic purposes. See, W. Paul in F. Yonezawa (Ed.), Fundamental Physics of Amorphous Semiconductors, Springer Series in Solid State Sciences, Volume 25 (1981), page 72 et seq.